Optical processing apparatus and optical processing method

ABSTRACT

In an annealing process in which laser light is irradiated to a semiconductor thin film, a refractive index of the semiconductor thin film after laser light irradiation is measured and conditions for the next laser light irradiation are adjusted based on the measured refractive index value. For example, laser light irradiation conditions are adjusted so that semiconductor thin films always have the same refractive index. As a result, the annealing can be performed under the same conditions at every laser light irradiation even if the laser light irradiation conditions vary unavoidably.

This application is a Continuation of application Ser. No. 08/451,648filed May 26, 1995, now U.S. Pat. No. 6,059,873.

BACKGROUND OF THE INVENTION

The present invention relates to a technique of evaluating a processingeffect of various processes using laser light. The invention alsorelates to a technique of relatively evaluating and controllingillumination energy of laser light.

“Low-temperature processes” are now being developed to manufacture aliquid crystal panel using polysilicon thin-film transistors (TFTs).This is intended to suppress the cost of a liquid crystal panel itselfby using a low-temperature process, which allows use of a glasssubstrate with which a large-size substrate can be obtained at a lowcost.

To realize a low-temperature process, the key subject is to crystallizean amorphous silicon film formed on a glass substrate by a heatingprocess of less than about 600° C., a temperature range in which theglass substrate can endure. There is known a low-temperature process inwhich an amorphous silicon film is formed on a glass substrate by CVDand converted to a crystalline silicon film by illumination with excimerlaser light. In this process, the amorphous silicon film is crystallizedby instantaneously rendering the surface and its vicinity of theamorphous silicon film into a molten state.

A crystalline silicon film that has been crystallized by illuminationwith laser light, particularly excimer laser light, is advantageous inthat it is close and has superior electrical characteristics. Further, asubstrate receives very little thermal damage. However, the excimerlaser light is associated with a problem that its illumination energy isunstable, resulting in a difficulty in keeping the optimum illuminatingcondition.

In the excimer laser, a particular gas is excited by subjectinghigh-frequency discharging to it, and electromagnetic waves areutilized, which are emitted when molecules of the gas transfer from theexcited state to the steady state. Therefore, there exists a problemoriginating from the principle that when laser oscillation is continued,increase of impurities in the gas or change in quality of the gas itselflowers the laser light output even with application of the samedischarge power. It is a general procedure to obtain a constant laserlight output by using a calibration table or the like. But this is notalways satisfactory. (For example, the illumination energy of laserlight is greatly varied by contamination or the like in a dischargechamber.)

It has been proved that the characteristics of a thin-film transistorproduced by using a crystalline silicon film that has been crystallizedby illumination with laser light approximately depend on theillumination energy of laser light. Therefore, if the illuminationenergy of laser light can be made constant or a desired value, athin-film transistor having intended characteristics can be obtained.This is not limited to the thin-film transistor, but also is widelyapplicable to other semiconductor devices that are produced by a processincluding laser light illumination.

There are several methods of evaluating the annealing effects of laserlight illumination on a semiconductor. Examples of these techniques aredisclosed in Japanese Unexamined Patent Publication Nos. Sho. 58-15943,Sho. 58-40331, and Hei. 1-16378.

In these methods, prescribed anneal effects of laser light illuminationon a semiconductor, particularly its crystallinity, are measured byRaman spectroscopy, to evaluate the annealing effects. However, theRaman spectroscopy has the following problems.

(1) Bad reproducibility of measurements.

(2) Use of a large-output laser such as an Ar laser causes a problem insafety.

(3) An expensive apparatus is needed.

(4) A measurement takes long time.

It is difficult to evaluate the flatness of a film surface by the Ramanspectroscopy, through the flatness of a film surface is an importantfactor of determining the characteristics of a thin-film transistormanufactured. Thus, a crystalline silicon film to be used for athin-film transistor having desired characteristics cannot be evaluatedsufficiently only by the Raman spectroscopy.

In the above circumstances, at present, in addition to theabove-described evaluation using the Raman spectroscopy, the flatness ofa film is evaluated by human eyes using an optical microscope or a SEM(scanning electron microscope).

As described above, at present, a crystalline silicon film is producedin the following manner, and a thin-film transistor having desiredcharacteristics is formed by using the crystalline silicon film thusproduced.

(A) In a process of crystallizing an amorphous silicon film by usingexcimer laser light, the optimum illumination condition of excimer laserlight is found experimentally. And it is tried to always perform laserlight illumination under the optimum condition.

(B) The optimum condition is set by evaluating the crystallinity of thefilm by the Raman spectroscopy and evaluating its flatness by visualobservation.

However, as described above, the illumination energy of excimer laserlight is liable to vary and it is difficult to control the illuminationenergy. As mentioned above as item (B), the evaluation of the effects oflaser light illumination depends on the two parameters of thecrystallinity evaluation by the Raman spectroscopy and the evaluation ofthe film flatness by visual observation. It is therefore difficult tocontrol, using the two parameters, the illumination power of excimerlaser light, which tends to gradually change, so that it is kept at theoptimum value.

SUMMARY OF THE INVENTION

The present invention is intended to attain at least one of thefollowing objects.

(1) To provide a technique capable of judging, on a realtime basis, theeffects of various processes using laser light, such as a process ofimproving the quality of a thin film and annealing of a thin film.

(2) To provide a technique capable of performing laser lightillumination while making control for always maintaining the optimumcondition in a process of improving the quality of a thin film andannealing of a thin film both using laser light.

(3) To provide a technique capable of easily evaluating thecrystallinity of a silicon thin film in a crystallization process of asilicon thin film using laser light.

(4) To provide a technique capable of easily evaluating thecrystallinity of a silicon thin film and the flatness of its surface ina crystallization process of a silicon thin film using laser light.

(5) To provide a technique of controlling the illumination energy oflaser light so that it is always kept close to a predetermined value.

According to one of principal aspects of the invention, there isprovided an optical processing method comprising the steps of:

forming a semiconductor thin film on a substrate having an insulativesurface;

irradiating laser light or high-intensity light onto the thin film;

measuring a refractive index of the thin film to which the laser lightor the high-intensity light has been irradiated; and

controlling an illumination energy of the laser light or thehigh-intensity light based on the measured refractive index.

In the above method, examples of the substrate having an insulativesurface are a glass substrate, a quartz substrate, other variousinsulative substrates, semiconductor substrates or conductor substrateson which an insulative film is formed, and substrates of other materialson which an insulative film is formed.

Examples of the thin film are an amorphous silicon film and acrystalline silicon film which are semiconductor thin films. Theconductivity type of a semiconductor is not limited specifically. Otherexamples of the thin film are thin films made of an oxide material, anitride material, a metal material, or an organic material, i.e., amaterial whose quality is changed by illumination with laser light orhigh-intensity light.

Examples of laser light are excimer laser light of KrF, ArF or XeCl.High-intensity light may have any necessary wavelength from theultraviolet range to the infrared range. A laser beam may have any shapesuitable for each use, such as a rectangular shape, a linear shape, apoint-like shape, or a planar shape.

An example of the method of measuring the refractive index of athin-film is a method using ellipsometry.

An example of the method of controlling the illumination energy of laserlight or high-intensity light is, in the case of excimer laser light, amethod of controlling the discharge output.

The above processing method is characterized by evaluating the effectsof the laser light illumination by measuring the refractive index ofsemiconductor thin film whose quality has been changed by theillumination with laser light. For example, a desired effect can alwaysbe obtained by controlling the laser light illumination energy so as toalways produce a particular refractive index. Or an effect of the laserlight illumination can be made within a certain range by causing therefractive index of a laser-light-irradiated semiconductor film to fallwithin a predetermined range.

According to another principal aspect of the invention, there isprovided an optical processing method comprising the steps of:

forming an amorphous silicon film on a substrate having an insulativesurface;

crystallizing the amorphous silicon film with the aid of at least oneelement for facilitating crystallization of the amorphous silicon film;

irradiating laser light or high-intensity light to the crystallizedsilicon film;

measuring a refractive index of the silicon film to which the laserlight or the high-intensity light has been irradiated; and

controlling an irradiation energy of the laser light or thehigh-intensity light based on the measured refractive index.

The above method is characterized in that the silicon film to beirradiated with laser light is a film that has been crystallized withthe aid of at least one element for facilitating crystallization. The atleast one element may be one or a plurality of elements selected fromNi, Pd, Pt, Cu, Ag, Au, In, Sn, Pb, As and Sb. In particular, remarkableeffects can be obtained when Ni is used. Specifically, an amorphoussilicon film can be crystallized to obtain a crystalline silicon film byintroducing the element for facilitating crystallization into theamorphous silicon film and the subjecting it to a heat treatment. Theheat treatment can be performed at a temperature lower more than 50° C.compared with the case where no catalyst element for facilitatingcrystallization is used. In addition, heating damage to the substrate(particularly a glass substrate) can be greatly reduced.

The at least one elements for facilitating crystallization may be one ora plurality of elements selected from the elements of VIII, IIIb, IVband Vb families.

According to still another principal aspect of the invention, there isprovided an optical processing apparatus comprising:

means for irradiating laser light or high-intensity light to a thinfilm; and

means for controlling irradiation energy of the laser light or thehigh-intensity light based on a refractive index of the thin film towhich the laser light or the high-intensity light has been irradiated.

In the above processing apparatus, an example of the means forcontrolling the irradiation energy of laser light or high-intensitylight is a mechanism of controlling discharge power of an excimer laser,for example.

In the above processing apparatus, the irradiation energy of laser lightor high-intensity light can be made equal or close to a predeterminedvalue, or within a predetermined range by controlling the irradiationenergy of laser light or high-intensity light so that the refractiveindex of the thin film becomes a predetermined value or falls within apredetermined range. Further, by repeating the above operation, therefractive index value can be gradually made close to a predeterminedvalue.

The irradiation energy of laser light can be evaluated in a relativemanner by measuring the refractive index of a thin film whose qualityhas been changed by irradiation with laser light. For example, theirradiation energy of laser light can always be made close to aparticular value by adjusting the laser light irradiation energy so asto always produce a constant refractive index. Therefore, even where theirradiation energy of laser light is liable to vary, the variation canbe made as small as possible by monitoring the irradiation energy valueusing the refractive index. In other words, by measuring the refractiveindex of a thin film whose quality is changed by the irradiation withlaser light, a variation of the laser light irradiation energy value canbe monitored and the refractive index can be caused to have apredetermined value or fall within a predetermined range. Further, thelaser light irradiation energy value can be caused to have apredetermined value or fall within a predetermined range. By utilizingthis fact, the effects of the laser light irradiation can be madepredetermined ones.

For example, FIG. 4 shows experimental data representing a relationshipbetween the irradiation energy density of laser light and the refractiveindex n of a silicon thin film whose crystallinity has been improved bythe laser light irradiation. Based on this graph, the refractive indexof a silicon thin film can be made close to a predetermined value byincreasing the laser light irradiation energy density in the nextirradiating operation when the refractive index n of a silicon thin filmis larger than a predetermined value, and decreasing the laser lightirradiation energy density in the next irradiating operation when therefractive index n of a silicon thin film is smaller than thepredetermined value.

With the above operation, even where the laser light irradiation energydensity is liable to vary, the variation can be recognized from therefractive index of a silicon film whose crystallinity has been improvedby the laser light irradiation, so that the laser light irradiationdensity can be so controlled as to always allow laser light irradiationat a predetermined energy density. Thus, the effects of laser lightirradiation can be made constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general configuration of a laser light irradiationapparatus;

FIG. 2 shows an optics that is disposed in the laser light illuminationapparatus;

FIG. 3 shows the principle of ellipsometry;

FIG. 4 is a graph showing a relationship between the irradiation energydensity of laser light and the refractive index of a crystalline siliconfilm obtained by the laser light irradiation;

FIGS. 5(A)-5(D) show manufacturing steps of a thin-film transistor;

FIGS. 6(A) to 6(C) show a flowchart of a step of forming a crystallinesilicon film by use of laser light; and

FIGS. 7(A) to 7(C) show a flowchart of laser light annealing in athin-film transistor forming step.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

This embodiment is directed to a system and a method of evaluatingstates (which are defined as a concept including the crystallinity andthe flatness of a film) of a silicon film that has been crystallized byillumination with excimer laser light.

First, a description will be made of an apparatus. FIG. 1 is aconceptual diagram of a laser annealing apparatus used in thisembodiment. KrF excimer laser light (wavelength: 248 nm; pulse width: 25ns) is emitted from an oscillator 2. Apparently other excimer lasers andlasers of other types can also be used.

Laser light emitted from the oscillator 2 are passed throughfull-reflection mirrors 5 and 6, amplified by an amplifier 3, andintroduced into an optics 4 via full-reflection mirrors 7 and 8. Thelaser beam, which has a rectangular shape of about 3×2 cm² beforeentering the optics 4, is shaped by the optics 4 into a long and narrowbeam (linear beam) of 10-30 cm in length and 0.1-1 cm in width. Thelaser light as output from the optics 4 has an energy of 1,000 mJ/shotat the maximum.

Shaping the laser light into such a long and narrow beam is to improveits workability. After being output from the optics 4, the linear beamis applied to a sample 11 via a full-reflection mirror 9. Since the beamis longer than the width of the sample 11, the entire sample 11 can beilluminated with laser light by moving the sample 11 only in onedirection. Therefore, a sample stage and driving device 10 can be madesimple in structure and maintained easily. Further, an alignmentoperation in setting the sample 11 can be performed easily.

The sample stage 10 to which laser light is irradiated is controlled bya computer, and is so designed as to move perpendicularly to the linearlaser light. A heater is incorporated below the stage 10 to keep thesample 11 at a predetermined temperature during the laser lightillumination.

FIG. 2 shows an optical path inside the optics 4. The profile of thelaser light is converted from a Gaussian distribution to a rectangulardistribution while the laser light passes through a cylindrical concavelens A, a cylindrical convex lens B, horizontal fly-eye lenses C and D.Further, the laser light is passed through cylindrical convex lenses Eand F, reflected by a mirror G (corresponding to the mirror 9 in FIG.1), converged by a cylindrical lens H, and finally irradiated to thesample 11.

FIG. 3 shows the principle of ellipsometry, which is to measure anapparent refractive index of a film. Based on the refractive indexobtained by the ellipsometry, the crystallinity and the flatness of asilicon film can be evaluated at the same time.

As shown in FIG. 3, in the ellipsometry, polarized light is madeobliquely incident on the surface of a measurement sample (thin film).The polarization state of the incident light is changed when it isreflected, and the amount of change depends on the thickness and therefractive index of the thin film. In the ellipsometry, the amount ofchange in the polarization state is measured and the thickness and therefractive index are determined from the amount of change thus measured.For example, if the thickness of a film is known, the refractive indexcan be determined.

In the following, an example of forming a crystalline silicon film on aglass substrate by illumination with laser light is described. First, aglass substrate (for instance, a Corning 7959 glass substrate) of 10cm×10 cm is prepared. A 2,000-Å-thick silicon dioxide film, which is toserve as an undercoat film for preventing impurities from being diffusedinto a semiconductor film from the glass substrate side, is formed onthe glass substrate by plasma CVD using TEOS as a material.

Then, an amorphous silicon film is deposited by plasma CVD.Alternatively, it may be deposited by low-pressure thermal CVD. In thisexample, the thickness of the amorphous silicon film is set at 500 Å.Apparently the thickness is not limited to this value, but may be set atany desired value. An oxide film is then formed on the surface of theamorphous silicon film by immersing the substrate into ammonium hydrateand keeping it at 70° C. for 5 minutes. Further, liquid-phase nickelacetate is coated on the surface of the amorphous silicon film by spincoating. The element of nickel serves to facilitate the crystallizationof the amorphous silicon film.

Next, hydrogen is removed from the amorphous silicon film by leaving itfor one hour in a nitrogen atmosphere of 450° C., to intentionally formdangling bonds in the amorphous silicon film, to thereby lower thethreshold energy in a subsequent crystallization step. The amorphoussilicon film is then crystallized by being subjected to a thermaltreatment of 550° C. for 4 hours in a nitrogen atmosphere. Nickelcontributes to reduction of the crystallization temperature to as low as550° C.

In the above manner, a crystalline silicon film can be formed on theglass substrate. Then, KrF excimer laser light (wavelength: 248 nm;pulse width: 25 ns) is irradiated to the crystalline silicon using theapparatus shown in FIG. 1. The crystallinity can be improved by theillumination with the laser light.

The laser beam is shaped into a rectangular beam by a beam-shapedconversion lens, to obtain a beam area of 125 mm×1 mm on a portion to beilluminated. A sample is placed on the stage 10, and the entire surfaceof the sample is illuminated by moving the stage 10 at a speed rate of 2mm/s. The laser light is irradiated in two stages: first at an energydensity of 200 mJ/cm² and then at 250-350 mJ/cm² (main illumination).The pulse rate is 30 pulses/s. During the laser light illumination, thesubstrate temperature is kept at 200° C. The illumination is performedin the air, i.e., no atmospheric control is conducted.

FIG. 4 shows a result of a measurement in which the laser lightillumination energy density of the second stage was varied from 250 to350 mJ/cm² and refractive indices of crystalline silicon films weremeasured by the ellipsometry (a semiconductor laser of a wavelength1,294 nm was used), whose principle has already been explained withreference to FIG. 3.

The “refractive index” mentioned above means a refractive index of acrystalline silicon film formed on a glass substrate as measured by theellipsometry. Stated in a more detailed manner, where a measurement filmis low in flatness, a refractive index as measured by the ellipsometrytends to be somewhat smaller than its true refractive index. Therefractive index mentioned above is an apparent one that includes such atendency. A smaller refractive index means that the film has a highdegree of crystallinity. Therefore, the smaller the refractive index,the higher the crystallinity of the film and the lower in its flatness.

From the above discussion, it is concluded that a film having a requiredlevel of crystallinity and allowable flatness exhibits a refractiveindex that is within a certain range. In other words, a crystallinesilicon film having a refractive index within a predetermined rangeshould have crystallinity higher than a certain level and flatnesswithin an allowable range.

It is understood from FIG. 4 that the refractive index of a crystallinesilicon film and the energy density of illumination laser light have aproportional relationship. Naturally, since the illumination energydensity of excimer laser light varies as described above, values on thehorizontal axis of FIG. 4 are considered to be relative ones.

Each experimental data plotted in FIG. 4 is an average of data of fivepoints on the film surface. A variation in measurement values of eachset of five points was within 2%, which indicates that the film has gooduniformity and the ellipsometry has high measurement accuracy.

As described above, in general, the refractive index measured by theellipsometry tends to decrease as the degree of crystallization becomeshigher or the surface flatness is more degraded. Utilizing thistendency, the inventors have invented the following laser processingmethod. The refractive index of a crystalline silicon film that has beensubjected to excimer laser light illumination is measured. If themeasured refractive index is smaller than a predetermined value, theillumination energy is reduced for the subsequent laser lightirradiating operations. Conversely, if the measured refractive index islarger than the predetermined value, the illumination energy isincreased. In this manner, crystalline silicon films can be obtainedwhich always have a refractive index equal to or close to thepredetermined value. That is, even if the absolute value of the laserlight illumination energy is varied, crystalline silicon films can beobtained which always have crystallinity of a predetermined level andallowable flatness.

For example, in the above constitution in which an amorphous siliconfilm is crystallized by a heat treatment with the aid of Ni and thecrystallinity of a resulting crystalline silicon film is improved by theillumination with KrF Excimer laser light, measurement results have beenobtained which indicate that the crystalline silicon film should have arefractive index of not more than 3.5 so as to have not only a surfacestate that is not so deteriorated as to impair performance of athin-film transistor produced by using the crystalline silicon film butalso a sufficiently large field-effect mobility for driving of athin-film transistor.

Measurements have proved that if the refractive index of a crystallinesilicon film after being subjected to laser light illumination is largerthan 3.5, the field-effect mobility of a thin-film transistor using sucha film is smaller than 100, and that if it is smaller than 3.5, thefield-effect mobility of a thin-film transistor is larger than 100.Therefore, for example, by forming a crystalline silicon film having arefractive index of 3.4, it can always provide a thin-film transistorhaving a field-effect mobility of more than 100.

However, it should be noted that if the refractive index is too small,the flatness of a film is so degraded that the film is not suitable fora thin-film transistor.

The ellipsometric measurement explained in this embodiment is very safe,and can be performed very easily in an extremely short time (severaltens of seconds). Therefore, the laser light illumination can always beperformed at an energy density having a predetermined absolute value bymeasuring the refractive index of a crystalline silicon film formed on asubstrate by the ellipsometry, for instance, after processing of eachsubstrate, and then controlling the illumination energy density forprocessing of the next substrate based on the thus-measured refractiveindex value. Thus, a variation in the effects of the laser lightillumination in processing substrates can be suppressed. This factenables mass-production of semiconductor devices, for instance,thin-film transistors, having predetermined characteristics. Although inthis embodiment a substrate is scanned by a linear laser beam, a laserbeam may be irradiated to a substrate so as to cover its entire surface.

Embodiment 2

This embodiment is directed to formation of a thin-film transistor byuse of the technique of the first embodiment. FIGS. 5(A)-5(D) showmanufacturing steps of a thin-film transistor. In this embodiment, aglass substrate 501 is used as the substrate. A 2,000-Å-thick silicondioxide film (not shown) is formed, as an undercoat film, on the surfaceof the glass substrate 501.

First, a 500-Å-thick amorphous silicon film 502 is formed on the glasssubstrate 501 by plasma CVD or low-pressure thermal CVD. Ultravioletlight is then applied to the surface of the amorphous silicon film 502in an oxygen atmosphere. After the surface is cleaned, a very thin oxidefilm is formed on the cleaned surface. A solution of nickel acetate isapplied to form a liquid film 503, and then spin-coated with a spinner500 (see FIG. 5(A)).

Next, the substrate is subjected to a heat treatment of 450° C. for onehour in an inert gas atmosphere to remove hydrogen from the amorphoussilicon film 502 and form a layer of a nickel-silicon compound on thesurface of the amorphous silicon film.

Then, the substrate is subjected to a heat treatment of 550° C. and fourhours in an inert gas atmosphere to diffuse nickel into the film andeffect its crystallization. Thus, a crystalline silicon film 504 isobtained. KrF excimer laser light is applied to the crystalline siliconfilm 504 by using the apparatus of FIG. 1 to improve its crystallinity.

The refractive index of the crystalline silicon film 504 is measured bythe ellipsometry after one substrate is processed. If the measuredrefractive index value is larger than a predetermined value (forinstance, 3.4), the setting value of the laser light illumination energyis increased. Conversely, if it is smaller than the predetermined value,the illumination energy is reduced. The next substrate is thereaftersubjected to the laser light illumination. In this manner, as substratesare processed consecutively, crystalline silicon films can always beobtained which have refractive indices equal to or close to thepredetermined refractive index; that is, crystalline silicon films in adesired state (having desired film quality) can be obtained.

FIG. 6 is a flowchart of the laser light illumination step. Parts (A) to(C) of FIG. 6 correspond to a process for one substrate. By feeding backa result of operation (C) to operation (A), correction can be made for avariation of the laser light illumination energy density that changesgradually. Thus, the variation can always be suppressed to a minimumvalue.

In the method of FIG. 6, the laser light illumination conditions areadjusted after processing of each substrate. Alternatively, therefractive index of a crystalline silicon film on the fifth substratemay be measured by the ellipsometry after processing of each set of, forinstance, five substrates, and the setting of the laser lightillumination energy density may be adjusted for the next substrate basedon the thus-measured refractive index.

After the crystalline silicon film 504 is obtained by the substep ofFIG. 5(B), patterning is performed to form an active layer of athin-film transistor, which layer is a semiconductor layer includingregions 507-509 (see FIG. 5(C)).

Subsequently, a 1,000-Å-thick silicon dioxide film 505 to serve as agate insulating film is formed by plasma CVD or sputtering. A gateelectrode 506 is then formed with a metal such as aluminum or a siliconsemiconductor heavily doped with impurities that impart a singleconductivity type. The source region 507 and the drain region 509 areformed by ion implantation (or plasma doping) of impurities that imparta single conductivity type using the gate electrode 506 as a mask. Thechannel forming region 508 is formed at the same time.

Referring to FIG. 7, laser light illumination is then performed tore-crystallize the source region 507 and drain region 509 which havebeen de-crystallized by the ion impact and to activate the implantedimpurities. To make the laser light illumination energy densityconstant, a separate sample substrate in the state of FIG. 5(B) isprepared, laser light is irradiated to the sample substrate each timethe processing of one substrate, for instance, is finished, and therefractive index of its illumination area is measured by theellipsometry. The laser light illumination conditions for the nextsubstrate are set based on the thus-measured refractive index value.

That is, the laser light illumination conditions are always controlledbased on the refractive index of a sample substrate. More specifically,the setting is changed to increase the laser light illumination energydensity if the refractive index of a sample substrate is larger than apredetermined value, and to decrease the illumination energy density ifthe refractive index is smaller than the predetermined value. In thismanner, correction is made for a variation of the laser lightillumination energy density every time the processing of one substrateis finished, so that the illumination energy density can be made closeto a particular value. As a result, the laser light illumination densitycan be made approximately constant for respective substrates, to alwaysprovide the same annealing conditions.

Next, an interlayer insulating film 510 is formed on the substrate ofFIG. 5(C) with an insulative material such as silicon oxide or acombination of silicon dioxide and silicon nitride. After formation ofholes, a source electrode 511 and a drain electrode 512 are formed. Agate electrode (not shown) is formed at the same time. The substrate isthen subjected to a heat treatment of 350° C. for one hour in a hydrogenatmosphere to neutralize dangling bonds in the active later. Thus, athin-film transistor is completed.

With the constitution of this embodiment, thin-film transistors can beformed by using crystalline silicon films that are always in a stateclose to a particular state, and the laser light annealing (of thesource and drain regions) can always be performed under conditions closeto particular ones. Therefore, thin-film transistors having almostidentical characteristics can be obtained.

Embodiment 3

This embodiment is directed to a technique of correcting the laser lightillumination conditions on a realtime basis. The data of FIG. 4represent a relationship between refractive indices (as measured by theellipsometry) of crystalline silicon films whose crystallinity has beenimproved by illumination with KrF excimer laser light and illuminationenergy densities (mJ/cm²) of laser light used. As mentioned above, theillumination energy densities of FIG. 4 do not represent actual energydensities of laser light used.

However, it is understood that the refractive index of acrystallinity-improved crystalline silicon film and the energy densityof illumination laser light has a relative relationship that isproportional as shown in FIG. 4. Therefore, by constantly controllingthe laser light illumination energy so as to provide a predeterminedrefractive index, a constant illumination energy density value canalways be obtained.

Therefore, where it is necessary to perform illumination using laserlight of a constant output, the illumination energy density of laserlight can be calibrated when necessary by separately preparing amonitoring crystalline silicon film and causing thelaser-light-irradiated crystalline silicon film to always have aconstant refractive index.

For example, a consideration will be made of a case where it isnecessary to irradiate laser light having predetermined energy to anillumination object body. In this case, a monitoring silicon film isseparately prepared, and the refractive index of the silicon filmcrystallized or crystallinity-improved by the laser light illuminationis measured for each necessary manufacturing step. The laser lightillumination energy is so changed that the measured refractive indexvalue becomes close to a predetermined value. As a result, a correction(calibration) can always be effected so as to make the laser lightillumination energy closer to a particular value every time an operationusing the monitoring silicon film substrate is performed. That is, thelaser light illumination energy can be made to fall within a certainrange.

The above constitution can be applied to various processing apparatusesusing laser light, such as an annealing apparatus, a working apparatusand a cutting apparatus.

As described above, according to the invention, various processingeffects caused by laser light illumination can be evaluated by measuringthe refractive index of a thin film whose quality is changed byillumination with laser light. Further, by measuring the refractiveindex of a thin film whose quality is changed by illumination with laserlight, the illumination energy value of laser light can be evaluated ina relative manner. This fact enables the laser light illumination energyto be controlled so as to become equal or close to a particular value.

By utilizing the present invention, a laser light annealing operation isallowed to always exhibit the same effects. Therefore, for example, itbecomes possible to manufacture thin-film transistors having almostidentical characteristics. It becomes possible to evaluate, easily andsimultaneously, the crystallinity of a crystalline silicon film forthin-film transistors and its flatness.

The present invention can be applied to manufacture of varioussemiconductors and control of illumination energy or power of laserlight.

What is claimed is:
 1. An optical processing method comprising the stepsof: preparing a semiconductor film over a substrate; irradiating a laserlight onto said semiconductor film, and controlling an irradiationenergy of said laser light based on a refractive index of saidsemiconductor film on which said laser light has been irradiated so thatthe refractive index of said semiconductor film is 3.5 or less.
 2. Anoptical processing method according to claim 1 wherein said laser lightis selected from the group consisting of KrF excimer laser light, ArFexcimer laser light and XeCl excimer laser light.
 3. An opticalprocessing method according to claim 1 wherein said irradiating step isperformed by relatively scanning said laser light with respect to saidsubstrate.
 4. An optical processing method according to claim 1 whereinsaid refractive index is measured by an ellipsometer.
 5. An opticalprocessing method according to claim 1 wherein a field effect mobilityof a thin film transistor using said semiconductor film having therefractive index of 3.5 or less is larger than 100 cm²/Vsec.
 6. Anoptical processing method comprising the steps of: preparing asemiconductor film formed over a substrate; irradiating a laser lightonto said semiconductor film; and controlling an irradiation energy ofsaid laser light based on a refractive index of said semiconductor filmon which said laser light has been irradiated, wherein said laser lightis repeatedly irradiated onto said semiconductor film until therefractive index of said semiconductor film becomes 3.5 or less.
 7. Anoptical processing method according to claim 6 wherein said laser lightis selected from the group consisting of KrF excimer laser light, ArFexcimer laser light and XeCl excimer laser light.
 8. An opticalprocessing method according to claim 6 wherein said irradiating step isperformed by relatively scanning said laser light with respect to saidsubstrate.
 9. An optical processing method according to claim 6 whereinsaid refractive index is measured by an ellipsometer.
 10. An opticalprocessing method according to claim 6 wherein a field effect mobilityof a thin film transistor using said semiconductor film having therefractive index of 3.5 or less is larger than 100 cm²/Vsec.
 11. Anoptical processing method comprising the steps of: preparing asemiconductor film formed over a substrate; irradiating a first laserlight onto said semiconductor film; and irradiating a second laser lightonto said semiconductor film, wherein an irradiation energy of saidsecond laser light is controlled so that a refractive index of saidsemiconductor film is 3.5 or less.
 12. An optical processing methodaccording to claim 11 wherein each of said first and second laser lightsis selected from the group consisting of KrF, excimer laser light, ArFexcimer laser light and XeCl excimer laser light.
 13. An opticalprocessing method according to claim 11 wherein said irradiating stepusing each of said first and second laser lights is performed byrelatively scanning said laser light with respect to said substrate. 14.An optical processing method according to claim 11 wherein saidrefractive index is measured by an ellipsometer.
 15. An opticalprocessing method according to claim 11 wherein a field effect mobilityof a thin film transistor using said semiconductor film having therefractive index of 3.5 or less is larger than 100 cm²/Vsec.
 16. Anoptical processing method comprising the steps of: preparing asemiconductor film formed over a substrate; irradiating a first laserlight onto said semiconductor film; measuring a first refractive indexof said semiconductor film on which said first laser light has beenirradiated; and irradiating a second laser light onto said semiconductorfilm, measuring a second refractive index of said semiconductor film onwhich said second laser light has been irradiated, wherein anirradiation energy of said second laser light is controlled based onsaid first refractive index; wherein said second refractive index ofsaid semiconductor film is 3.5 or less.
 17. An optical processing methodaccording to claim 16 wherein each of said first and second laser lightsis selected from the group consisting of KrF excimer laser light, ArFexcimer laser light and XeCl excimer laser light.
 18. An opticalprocessing method according to claim 16 wherein said irradiating stepusing each of said first and second laser lights is performed byrelatively scanning said laser lights with respect to said substrate.19. An optical processing method according to claim 16 wherein saidfirst and second refractive index are measured by an ellipsometer. 20.An optical processing method according to claim 16 wherein a fieldeffect mobility of a thin film transistor using said semiconductor filmhaving the refractive index of 3.5 or less is larger than 100 cm²/Vsec.21. An optical processing method comprising the steps of: preparing afirst semiconductor film over a first substrate; irradiating a firstlaser light onto said first semiconductor film; measuring a refractiveindex of said first semiconductor film; preparing a second semiconductorfilm formed over a second substrate; and irradiating a second laserlight onto said second semiconductor film, wherein an irradiation energyof said second laser light is controlled based on the refractive indexof said first semiconductor film so that the refractive index of saidsecond semiconductor film is 3.5 or less.
 22. An optical processingmethod according to claim 21 wherein each of said first and second laserlights is selected from the group consisting of KrF excimer laser light,ArF excimer laser light and XeCl excimer laser light.
 23. An opticalprocessing method according to claim 21 wherein said irradiating stepusing said first light is performed by relatively scanning said firstlaser light with respect to said first substrate.
 24. An opticalprocessing method according to claim 21 wherein said irradiating stepusing said second light is performed by relatively scanning said secondlaser light with respect to said second substrate.
 25. An opticalprocessing method according to claim 21 wherein said refractive index ismeasured by an ellipsometer.
 26. An optical processing method accordingto claim 21 wherein a field effect mobility of a thin film transistorusing said semiconductor film having the refractive index of 3.5 or lessis larger than 100 cm²/Vsec.
 27. An optical processing method comprisingthe steps of: preparing a semiconductor film formed over a substrate;irradiating a first laser light onto said semiconductor film; andirradiating a second laser light onto said semiconductor film, whereinan irradiation energy of said second laser light is larger than that ofsaid first laser light, wherein a refractive index of said semiconductorfilm on which said second laser light has been irradiated is 3.5 orless.
 28. An optical processing method according to claim 27 whereineach of said first and second laser lights is selected from the groupconsisting of KrF excimer laser light, ArF excimer laser light and XeClexcimer laser light.
 29. An optical processing method according to claim27 wherein said irradiating step using each of said first and secondlaser lights is performed by relatively scanning each of said laserlights with respect to said substrate.
 30. An optical processing methodaccording to claim 27 wherein said refractive index is measured by anellipsometer.
 31. An optical processing method according to claim 27wherein a field effect mobility of a thin film transistor using saidsemiconductor film having the refractive index of 3.5 or less is largerthan 100 cm²/Vsec.
 32. An optical processing method according to claim 1wherein said laser light has a rectangular-shaped cross section at saidsubstrate.
 33. An optical processing method according to claim 6 whereinsaid laser light has a rectangular-shaped cross section at saidsubstrate.
 34. An optical processing method according to claim 11wherein each of said first and second laser lights has arectangular-shaped cross section at said substrate.
 35. An opticalprocessing method according to claim 16 wherein each of said first andsecond laser lights has a rectangular-shaped cross section at saidsubstrate.
 36. An optical processing method according to claim 21wherein said first laser light has a rectangular-shaped cross section atsaid first substrate.
 37. An optical processing method according toclaim 21 wherein said second laser light has a rectangular-shaped crosssection at said second substrate.
 38. An optical processing methodaccording to claim 27 wherein each of said first and second laser lightshas a rectangular-shaped cross section at said substrate.